Magnetic recording medium and method for manufacturing same

ABSTRACT

A magnetic recording medium has a non-magnetic under-layer, a magnetic layer, a protective film and a liquid lubricant layer sequentially laminated on a non-magnetic substrate. The magnetic layer has a multi-layer structure laminated with two or more magnetic layer components, each of the magnetic layer components having ferromagnetic grains and non-magnetic grain boundaries surrounding the grain. The resulting magnetic recording medium has a granular magnetic layer exhibiting very high Hc accompanying high density of magnetic recording, while decreasing the amount of platinum needed for attaining the high Hc, and reducing media noise accompanying the high recording density.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a magnetic recording mediummounted on magnetic recording devices, such as an external memory deviceof a computer. The present invention further relates to a method formanufacturing such a recording medium.

BACKGROUND OF THE INVENTION

[0002] In recent years there has been a requirement for a magneticrecording medium with higher recording density and lower noise. Variousconventional compositions and structures of a magnetic layer andmaterials for a non-magnetic under-layer and a seed layer have beenproposed. In particular, a magnetic layer called the granular magneticlayer has been proposed having a structure in which a ferromagneticgrain is surrounded by non-magnetic non-metallic substance, such asoxide or nitride.

[0003] Japanese Unexamined Patent Application Publication No. H8-255342, for example, discloses attaining low noise by forming a granularrecording layer in which ferromagnetic grains are dispersed in anon-magnetic film. This is accomplished by a method comprising steps ofsequentially depositing a non-magnetic film, a ferromagnetic film and anon-magnetic film on a non-magnetic substrate, and heat-treating thelaminate. For this type of conventional magnetic layer, cobalt or analloy containing cobalt as a main component is used. A metal, oxide,nitride, carbon or carbide is used for the non-magnetic film.

[0004] U.S. Pat. No. 5,679,473 discloses that a granular recording film,in which each magnetic grain is surrounded by a non-magnetic oxide andseparated with each other, can be formed by means of RF (radiofrequency) sputtering using a CoNiPt target added with an oxide, such asSiO₂. Low noise is achieved by such a conventional recording film.

[0005] The low noise achieved in the above recording film is consideredto be achieved by the following reason. Since each of the magneticgrains in this granular magnetic film is physically separated by a grainboundary of non-magnetic non-metallic phase, magnetic interactionbetween the magnetic grains is reduced and formation of the magneticdomain wall with a zigzag shape at the transition region of a recordingbit is suppressed.

[0006] Noises of a recording medium are caused by fluctuation ofmagnetization due to magnetic interaction between magnetic grains thatconstitute the medium, and the size of the grain. In order to maintainhigh SNR keeping up with enhancement of the recording density, it isnecessary to hold the number of magnetic grains per bit cell greaterthan a certain value. In other words, minimization of the size of themagnetic grain is required. However, in the situation where largeexchange interaction arises between the magnetic grains, theminimization of magnetic grains frequently does not necessarily meanminimization of unit of reversed magnetization. Therefore, it is alsonecessary to suppress the exchange interaction between the grains forminimizing the unit of reversed magnetization itself that is representedby an activation magnetic moment. Further in the minimization, themagnetic grain itself must have a relatively large value of energy ofmagnetic anisotropy so that a superparamagnetic state does not occur andthe magnetic characteristic essential for high resolution recording,that is, a large Hc/Mrt value, can be obtained. The objective aimed atby the granular structure, in which magnetic grains having high energyof magnetic anisotropy are dispersed in a non-magnetic matrix, is thatthe above-described rigorous requirements are met for attaining highSNR.

[0007] In the conventionally used Co—Cr alloy magnetic film, chromium issegregated from a cobalt alloy magnetic grain towards a grain boundary,so as to reduce magnetic interaction between the magnetic grains. On theother hand, in the granular magnetic layer, the grain boundary phase iscomposed of a non-magnetic non-metallic substance, which segregateseasier than the conventional chromium. Consequently, isolation ofmagnetic grains is easily enhanced. In the conventional Co—Cr alloymagnetic layer, heating the substrate up to 200° C. is essential forsufficient segregation of chromium when laminating the layer. Thegranular magnetic layer has the advantage that the non-magneticnon-metallic substance segregates even in lamination without heating.

[0008] However, a magnetic recording medium having a granular magneticfilm requires addition of relatively large amount of platinum to thecobalt alloy to attain desired magnetic characteristic, in particular,high coercive force Hc. To achieve a coercive force of 2,800 Oe in agranular magnetic film, as high as 16 at % of platinum is commonlyneeded, while in the conventional CoCr alloy magnetic film, only 8 at %of platinum is required for obtaining the same value of the coerciveforce Hc. With the growing density of magnetic recording in recentyears, very high coercive force of higher than 3,200 Oe is becomingnecessary. As a result, the granular magnetic film that requires largeamount of expensive platinum has brought about a problem of rising ofmanufacturing cost. In addition, more reduction of the media noise isdemanded accompanying with enhancement of the recording density.

[0009] Moreover, with respect to crystal growth at a low thicknessstage, that is an initial growth stage, the granular magnetic layer isdisordered and a clear granular structure is not formed. This situationis the main cause of deterioration in magnetic characteristics andelectromagnetic conversion characteristics in the low Br δ region, Br 67being a product of remanent magnetic flux density and film thickness. Inthe future trend for the magnetic layer to become thinner, accompaniedby a higher recording density, this deterioration of the magneticcharacteristics and the electromagnetic conversion characteristics atthe initial growth stage of the granular magnetic layer are difficultproblems to solve.

[0010] Although the non-magnetic non-metallic substance in the granularmagnetic layer on a substrate segregates even in unheated lamination,in-plane orientation of magnetization in the magnetic layer is difficultto attain. An isotropic or random orientation medium is liable to beformed.

OBJECTS AND SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a magneticrecording medium which overcomes the foregoing problems.

[0012] It is another object of the present invention to provide amagnetic recording medium having a granular magnetic layer that exhibitsvery high Hc accompanying high density of magnetic recording, whiledecreasing the amount of platinum which is needed for attaining the highHc, and reducing media noise accompanying the high recording density.

[0013] It is a further object of the present invention to provide amethod for manufacturing a magnetic recording medium which overcomes theforegoing problems.

[0014] The inventors of the present invention have made rigorous studiesfor effectively giving desired alignment to the granular magnetic layerand for achieving high Hc, low noise, and low cost in the granularmagnetic film. The inventors have found that, in order to obtain desiredalignment in the magnetic layer, alignment in the layer below themagnetic layer has to be controlled and the magnetic layer needs toepitaxially grow on this controlled layer. It is further revealed thatdefinitely higher Hc and lower noise can be achieved, as compared toconventional magnetic recording medium that has a continuously depositedmagnetic layer, that is, a magnetic layer composed of a single magneticlayer component, when a process for laminating the magnetic layer isdivided into a plurality of steps and the magnetic layer is formed witha plurality of magnetic layer components.

[0015] Advantageously, oxide layers are provided on and beneath each ofthe magnetic layer components in the multi-layered magnetic layer.

[0016] Specifically, the present invention provides two types ofmagnetic recording media, depending on the structure of a magnetic layerof the medium. The first magnetic recording medium of the presentinvention comprises a non-magnetic under-layer, a magnetic layer, aprotective film and a liquid lubricant layer sequentially laminated on anon-magnetic substrate. The magnetic layer has a multi-layered structurebeing laminated with two or more magnetic layer components, each ofwhich consists of ferromagnetic grains and non-magnetic grain boundariessurrounding the grain.

[0017] Advantageously, the composition of the magnetic layer componentsin the magnetic layer is different from one another.

[0018] The second magnetic recording medium of the invention comprises anon-magnetic under-layer, a magnetic layer, a protective film, and aliquid lubricant layer sequentially laminated on a non-magneticsubstrate. The magnetic layer comprises two or more magnetic layercomponents and three or more oxide layers, each of the magnetic layercomponents consists of ferromagnetic grains and non-magnetic grainboundaries surrounding the grains. The magnetic layer components and theoxide layers are alternately laminated such that the top layer and thebottom layer of the magnetic layer are oxide layers.

[0019] Advantageously, the non-magnetic grain boundary in the magneticlayer components in the first and second magnetic recording media iscomposed of oxide or nitride of at least one element selected from thegroup consisting of Cr, Co, Si, Al, Ti, Ta, Hf and Zr.

[0020] Advantageously, the non-magnetic under-layer of theabove-described magnetic recording medium is composed of chromium orchromium alloy. The non-magnetic substrate may be made preferably ofcrystallized glass, chemically strengthened glass or plastic.

[0021] A method for manufacturing the above-described first magneticrecording medium of the invention the following steps:

[0022] (1) laminating a non-magnetic under-layer on a non-magneticsubstrate,

[0023] (2) laminating a magnetic layer on the under-layer by depositinga plurality of magnetic layer components, each of the componentscomprising ferromagnetic grains and grain boundaries surrounding thegrains,

[0024] (3) laminating the protective film on the magnetic layer, and

[0025] (4) laminating the liquid lubricant layer on the protectivelayer.

[0026] Advantageously, the step for laminating the magnetic layer is astep for depositing a plurality of magnetic layer components each havinga composition different from that of the other components.

[0027] A method for manufacturing the second magnetic recording mediumof the invention comprises the following steps:

[0028] (1) laminating a non-magnetic under-layer on a non-magneticsubstrate,

[0029] (2) laminating a magnetic layer on the under-layer by

[0030] (i) exposing to an atmosphere of oxygen-containing gas andforming an oxide layer on the surface exposed to oxygen,

[0031] (ii) depositing a magnetic layer component comprisingferromagnetic grains and grain boundaries surrounding the grains,

[0032] (iii) repeating the procedures (i) and (ii) desired times, and

[0033] (iv) exposing to an atmosphere of oxygen-containing gas andforming an oxide layer on the surface exposed to oxygen,

[0034] (3) laminating a protective film on the magnetic layer, and

[0035] (4) laminating a liquid lubricant layer on the protective layer.

[0036] The above-described method for manufacturing a magnetic recordingmedium according to the invention allows performance of steps (1) to (4)without heating the non-magnetic substrate in advance.

[0037] The above, and other objects, features and advantages of thepresent invention will become apparent from the following descriptionread in conjunction with the accompanying drawings, in which likereference numerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a schematic cross-sectional view of an example of amagnetic recording medium according to the present invention.

[0039]FIG. 2(a) is a schematic cross-sectional view of a magnetic layerof a first magnetic recording medium according to the present inventioncomprising a plurality of magnetic layer components.

[0040]FIG. 2(b) is a schematic cross-sectional view of a magnetic layerof a second magnetic recording medium according to the present inventioncomprising a plurality of magnetic layer components and a plurality ofoxide layers disposed on and beneath each of the magnetic layercomponents.

[0041]FIG. 3 is a graph showing the dependence of coercive force Hc onthe number of magnetic layer components of the magnetic layer.

[0042]FIG. 4 is a graph showing the dependence of SNR on the number ofmagnetic layer components of the magnetic layer.

DETAILED DESCRIPTION OF THE INVENTION

[0043] Now, aspects of some preferred embodiments of the invention willbe described referring to FIG. 1, FIG. 2(a) and FIG. 2(b).

[0044] The magnetic recording medium of FIG. 1 has a structure in whicha non-magnetic under-layer 2 a, magnetic layer 3 and a protective film 4are sequentially formed on a non-magneticmagnetic substrate 1. A liquidlubricant layer 5 is formed on the laminate. The magnetic layer shown inFIG. 2(a) has a three-layer structure in which magnetic layer components3 a, 3 b and 3 c are laminated. The magnetic layer shown in FIG. 2(b)has a laminate structure in which each of magnetic layer components 3 a,3 b and 3 c is sandwiched by two of the oxide layers 3 a′, 3 b′, 3 c′and3 d′. The resulting laminate is layered in the sequence 3 a′, 3 a, 3 b′,3 b, 3 c′, 3 c and 3 d′.

[0045] The first magnetic recording medium of the present invention willbe described first.

[0046] Non-magnetic substrate 1 may be made of NiP-plated aluminumalloy, strengthened glass or crystallized glass as in the conventionalmagnetic recording medium. In addition, a substrate made by injectionmolding polycarbonate, polyolefin or other resins may also be used sinceheating the substrate is not required by the production process of theinvention.

[0047] Non-magnetic under-layer 2 is formed on non-magneticmagneticsubstrate 1 by any conventional means, such as electron-beam evaporationor sputtering. Non-magnetic under-layer 2 is composed ofnon-magneticmagnetic substance including NiAl and Cr. Chromium orchromium alloy is preferably used for under-layer 2. Preferable chromiumalloys include CrMo, CrTi, CrV and CrW. The thickness ofnon-magneticmagnetic under-layer 2 is preferably in the range from 5 nmto 50 nm for obtaining optimum magnetic characteristics and anelectromagnetic conversion characteristic.

[0048] Magnetic layer 3 is formed on non-magneticmagnetic under-layer 2.The structure of magnetic layer 3 of a first magnetic recording mediumis a multi-layer structure composed of a plurality of magnetic layercomponents 3 a, 3 b and 3 c that are laminated by a magnetic layerdeposition process divided into a plurality of steps. Each of magneticlayer component 3 a, 3 b and 3 c is a so-called granular magnetic layerthat comprises ferromagnetic grains and non-magneticmagnetic grainboundaries surrounding the grains. The non-magneticmagnetic grainboundary are composed of oxide or nitride of metals and silicon. Suchstructure of magnetic layer components 3 a, 3 b and 3 c may be obtained,for example, by deposition employing sputtering method using a target offerromagnetic metal containing oxide that composes the non-magneticgrain boundary. Alternatively, magnetic layer components 3a, 3 b and 3c, having granular structure, may be obtained by deposition employingreactive sputtering in oxygen-containing argon gas using a target offerromagnetic metal.

[0049] As material for composing the ferromagnetic grains, CoPt alloy isused preferably. Specifically, a CoPt alloy added with at least anelement selected from the group consisting of chromium, nickel andtantalum is favorable for reducing media noise. As material forcomposing the non-magneticmagnetic grain boundaries, an oxide or anitride of at least one element selected from the group consisting ofCr, Co, Si, Al, Ti, Ta, Hf and Zr is particularly favorable for forminga stable granular structure.

[0050] Thickness of magnetic layer 3, having a multi-layer structure, isnecessary to be such a value that provides enough head reproductionoutput when reproducing a record. The total thickness is desirable to benearly equal to the thickness needed by a conventional one-layeredcontinuous film.

[0051] Though the magnetic layer components may have the samecomposition, better characteristics can be obtained by differentcomposition, which may be performed by changing the concentration of theoxide or nitride, for example.

[0052] Though the magnetic layer illustrated in FIG. 2(a) is composed ofthree layers of magnetic layer component, the magnetic layer of themagnetic recording medium of the present invention is only necessary tobe formed with two or more layers of magnetic layer components.

[0053] Protective film 4 and liquid lubricant layer 5 are sequentiallyformed on magnetic layer 3. Protective film 4 and liquid lubricant layer5 may be conventional ones. For example, a thin film mainly composed ofcarbon may be used for protective film 4 and perfluoropolyetherlubricant maybe used for liquid lubricant layer 5. Protective film 4maybe laminated by a common method, such as sputtering, and liquidlubricant layer 5 may be formed by a common method, such as coating witha liquid lubricant.

[0054] Thus, a first magnetic recording medium of the invention isobtained.

[0055] The second magnetic recording medium is similar to the firstmagnetic recording medium described above except that the constructionof the magnetic layer is different from the construction of the magneticlayer of the first magnetic recording medium as shown in FIG. 2(b).Magnetic layer 3 formed in the second magnetic recording medium has astructure in which each of magnetic layer components 3 a, 3 b and 3 c issandwiched by two of oxide layers 3 a′, 3 b′, 3 c′ and 3 d′ as shown inFIG. 2(b). The magnetic layer components are formed in the same manneras in the first magnetic recording medium. The oxide layers are formedby oxidizing the surface of each of the layer components exposed to anoxygen-containing gas atmosphere. Specifically, a process of exposingthe media to an oxygen-containing gas atmosphere, for example, Ar-10% O₂gas, is performed before laminating the magnetic layer component 3 a andafter laminating each of the magnetic layer components 3 a, 3 b and 3 c.

[0056] Materials of the magnetic layer components and thickness ofmagnetic layer 3 formed in the second magnetic recording medium are thesame as those described with respect to magnetic layer 3 of the firstmagnetic recording medium.

[0057] As in magnetic layer 3 of the first magnetic recording medium,better characteristics can be obtained by different composition in themagnetic layer components. These changes may be performed, for example,by changing the concentration of the oxide or nitride, though the samecomposition is possible.

[0058] Though thickness of each of the magnetic layer components andoxide layers is not limited to special range, thickness of magneticlayer 3, that is, the total of the thickness of all magnetic layercomponents and oxide layers, is preferably nearly equal to the thicknessrequired by the conventional magnetic layer of a continuous film.

[0059] Though the magnetic layer illustrated in FIG. 2(b) is formed byalternately laminating three magnetic layer components and four oxidelayers, the magnetic layer of the magnetic recording medium of thepresent invention is only necessary to be constructed with two or moremagnetic layer components and three or more oxide layers. The number ofoxide layers is larger by one layer than the number of magnetic layercomponents, wherein the magnetic layer components and the oxide layersare alternately laminated such that the top layer and the bottom layerof magnetic layer 3 are the oxide layers.

[0060] Thus, a magnetic recording medium of the present invention isobtained that allows high Hc, low noise and low cost. The followingdescribes effects of multi-layer structure and oxide layer of themagnetic layer that are special features of the magnetic recordingmedium of the invention.

[0061] In usual longitudinal recording, large vertical component ofmagnetization causes noises as compared with the case in which themagnetization is aligned exactly in the direction of horizontal plane. Athick magnetic layer generally gives rise to vertical alignment ofmagnetization. Therefore, the magnetic layer of the present invention ismulti-layered. When each of the formed films, which are magnetic layercomponents, is made thin, in-plane orientation of the component film ispromoted. Thus, the vertical component of magnetization in the magneticlayer as a whole is decreased, which realizes noise reduction, leadingto high Hc.

[0062] The granular magnetic layer laminated with a plurality ofmagnetic layer components in the present invention promotes epitaxialgrowth of the grain of the ferromagnetic crystal and the grain boundaryof oxide in the uppermost magnetic layer component. Moreover,improvement of cystallinity and diminishing grain size of the granularmagnetic layer component itself directly under the uppermost magneticlayer component can also be accomplished, resulting in better control ofthe alignment in the magnetic layer.

[0063] As described earlier, it is more favorable to laminate magneticlayer components having different composition by changing theconcentration of the oxide or nitride.

[0064] More specifically, when the quantity of added oxide or nitride isincreased in a granular magnetic film for accelerating grain boundarysegregation, fine grain size is attained, which is considered to benecessary for noise reduction. On the other hand, the increased amountof oxide or nitride causes difficulty in the epitaxial growth from theunder-layer.

[0065] Consequently, the uppermost magnetic layer component of themagnetic layer is given a composition to exhibit excellent magneticcharacteristics and electromagnetic conversion characteristics. Thelower magnetic layer components, on the other hand, are provided forpromoting orderly epitaxial growth of the uppermost magnetic layercomponent and lattice matching with a layer beneath the magnetic layer,which is an under-layer, for example. Namely, the uppermost component ofthe granular magnetic layer is formed as a granular film containingincreased amount of oxide or nitride to achieve noise reduction, whilethe lower magnetic layer components are formed as granular filmscontaining less amount of oxide or nitride to accelerate epitaxialgrowth and containing increased or decreased amount of platinum andchromium. Since increase in platinum or chromium content in a CoCr alloyincreases lattice constants, the amount of the elements areappropriately varied, taking into consideration the composition of theuppermost component of the granular magnetic layer and the misfits inthe layers formed under the magnetic layer, for example, theunder-layer.

[0066] Further, lattice matching of misfit is facilitated more readilyby the layered structure of the magnetic layer component formed betweenthe layer just beneath the magnetic layer, an under-layer, here, and theuppermost magnetic layer component of the magnetic layer.

[0067] Since the material that bears the segregation of theferromagnetic grain to the grain boundary in the granular magnetic filmis an oxide, oxygen quantity profoundly affects promotion of segregationstructure. In the second magnetic recording medium of the invention, theoxygen is supplied by the oxide added to the target for the magneticlayer and also by the oxide layers provided on and beneath each magneticlayer component.

[0068] In the lamination process of the magnetic layer components of themagnetic layer of the second magnetic recording medium of the invention,each magnetic layer component is exposed to an oxygen-containingatmosphere so as to form oxide layers on and beneath each magnetic layercomponent. The provision of the oxide layers more effectively suppliesoxygen to the granular magnetic film to promote segregation structure.As a result, the interaction between the magnetic grains is suppressed,leading to noise reduction and Hc enhancement.

[0069] Consequently, high Hc and low noise is attained even if smalleramount of platinum than conventional is contained, which results in costreduction.

[0070] A magnetic recording medium of the invention havingabove-described lamination structure allows to achieve high Hc and lowmedia noise even if the manufacturing procedure omits a step for heatingthe substrate, the heating step being involved in the production of aconventional magnetic recording medium. Accordingly, reduction ofmanufacturing cost is achieved due to the simplification of theproduction procedure.

[0071] Further, plastic with less cost may be used for the substrate aswell as conventional aluminum and glass.

EXAMPLES

[0072] The present invention will be described more in detail referringto examples and comparative examples of the magnetic recording medium.

Example 1

[0073] A chemically strengthened glass substrate with smooth surface(N-10 glass substrate manufactured by Hoya Corp.) was used for asubstrate. After cleaning, the substrate was introduced into asputtering apparatus. A non-magneticmagnetic under-layer 2 havingthickness of 15 nm was formed of Cr-20 at% Mo by a dc magnetronsputtering method using a target of Mo-containing Cr alloy under anargon gas pressure of 50 mTorr without heating the substrate.

[0074] Then, a granular magnetic layer component 3 a having thickness of10 nm and the same composition as the target was formed by an RFsputtering method using a target of Co-10 at % Cr-14 at % Pt containing7 mol % of SiO₂ under an argon gas pressure of 30 mTorr. Subsequently,another granular magnetic layer component 3 b having thickness of 10 nmwas formed under the same conditions as in the component 3 a. Thus, thedouble-layered magnetic layer 3 was formed.

[0075] On the magnetic layer 3, a carbon protective film 4 of 10 nmthickness was deposited by a sputtering method, and then, the resultedarticle was taken out from the vacuum of the sputtering apparatus.

[0076] A liquid lubricant layer 5 having thickness of 1.5 nm was formedby applying liquid lubricant of perfluoropolyether on the carbonprotective film 4.

[0077] Thus, a magnetic recording medium as shown in FIG. 1 wasproduced.

[0078] In the above-described laminating process, heating thenon-magneticmagnetic substrate 1 in advance was not performed.

[0079] Concerning magnetic characteristics of the produced magneticrecording medium, the coercive force Hc and the product Br δ of remanentmagnetic flux density and film thickness were measured using a vibratingsample magnetometer VSM. Concerning electromagnetic conversioncharacteristics, regeneration output of solitary regeneration wave TAA,and media noise and SNR (signal-to-noise ratio) at track recordingdensity of 120 kFCI were measured using a GMR head on a spinning standtester.

[0080] Table 1 shows the composition of the laminate structure and Table2 gives the measured characteristics.

Example 2

[0081] A magnetic recording medium as shown in FIG. 1 was produced inthe same manner as in Example 1 except that a magnetic layer 3 shown inFIG. 2(a) having total thickness of 20 nm was provided by laminatingthree magnetic layer components 3 a, 3 b and 3 c, each having thicknessof about 6.7 nm.

[0082] The magnetic characteristics and electromagnetic conversioncharacteristics on the obtained magnetic recording medium were measuredin the same manner as in Example 1.

[0083] The composition of the laminate structure of this example isshown in Table 1 and the measured characteristics are given in Table 2.

Example 3

[0084] A magnetic recording medium as shown in FIG. 1 was produced inthe same manner as in Example 1 except that a magnetic layer 3 havingtotal thickness of 20 nm was provided by laminating four magnetic layercomponents, each having thickness of 5 mn.

[0085] The magnetic characteristics and electromagnetic conversioncharacteristics on the obtained magnetic recording medium were measuredin the same manner as in Example 1.

[0086] The composition of the laminate structure of this example isshown in Table 1 and the measured characteristics are given in Table 2.

Example 4

[0087] A magnetic recording medium as shown in FIG. 1 was produced inthe same manner as in Example 2 except that a magnetic layer 3 shown inFIG. 2(b) was formed that comprises oxide layers 3 a′, 3 b′, 3 c′ and 3d′ on and beneath each of the magnetic layer components by exposing toAr-10% O₂ gas atmosphere at 10 mTorr for 10 seconds before depositingthe magnetic layer component 3 a and after depositing each of themagnetic layer components 3 a, 3 b and 3 c.

[0088] The magnetic characteristics and electromagnetic conversioncharacteristics on the obtained magnetic recording medium were measuredin the same manner as in Example 1.

[0089] The composition of the laminate structure of this example isshown in Table 1 and the measured characteristics are given in Table 2.

Comparative Example 1

[0090] A magnetic recording medium was produced in the same manner as in

Example 1 except that the magnetic layer 3 was composed of a singlemagnetic layer component having thickness of 20 nm.

[0091] The magnetic characteristics and electromagnetic conversioncharacteristics on the obtained magnetic recording medium were measuredin the same manner as in Example 1.

[0092] The composition of the laminate structure of this example isshown in Table 1 and the measured characteristics are given in Table 2.

Example 5

[0093] A magnetic recording medium as shown in FIG. 1 was produced inthe same manner as in Example 1 except that a target having compositionof Co-10 at % Cr-14 at % Pt added with 12 mol % of SiN was used andgranular magnetic layer components 3 a and 3 b having the samecomposition as that of the target were formed, for obtaining magneticlayer 3

[0094] The magnetic characteristics and electromagnetic conversioncharacteristics on the obtained magnetic recording medium were measuredin the same manner as in Example 1.

[0095] The composition of the laminate structure of this example isshown in Table 1 and the measured characteristics are given in Table 2.

Example 6

[0096] A magnetic recording medium as shown in FIG. 1 was produced inthe same manner as in Example 2 except that the composition of thetarget for forming the magnetic layer components of the magnetic layer 3as shown in FIG. 2(a) was Co-10 at % Cr-14 at % Pt added with 12 mol %of SiN.

[0097] The magnetic characteristics and electromagnetic conversioncharacteristics on the obtained magnetic recording medium were measuredin the same manner as in Example 1.

[0098] The composition of the laminate structure of this example isshown in Table 1 and the measured characteristics are given in Table 2.

Example 7

[0099] A magnetic recording medium as shown in FIG. 1 was produced inthe same manner as in Example 4 except that the composition of thetarget for forming the magnetic layer components of the magnetic layer 3as shown in FIG. 2(b) was Co-10 at % Cr-14 at % Pt added with 12 mol %of SiN.

[0100] The magnetic characteristics and electromagnetic conversioncharacteristics on the obtained magnetic recording medium were measuredin the same manner as in Example 1.

[0101] The composition of the laminate structure of this example isshown in Table 1 and the measured characteristics are given in Table 2.

Comparative Example 2

[0102] A magnetic recording medium was produced in the same manner as inExample 5 except that the magnetic layer 3 was composed of a singlemagnetic layer component having thickness of 20 nm.

[0103] The magnetic characteristics and electromagnetic conversioncharacteristics on the obtained magnetic recording medium were measuredin the same manner as in Example 1.

[0104] The composition of the laminate structure of this example isshown in Table 1 and the measured characteristics are given in Table 2.TABLE 1 number of thickness of magnetic magnetic layer layer compo-component oxide composition nents (nm) layer Example 1Co-10Cr-14Pt-7SiO₂ 2 10 none Example 2 Co-10Cr-14Pt-7SiO₂ 3 6.7 noneExample 3 Co-10Cr-14Pt-7SiO₂ 4 5 none Example 4 Co-10Cr-14Pt-7SiO₂ 3 7provided Comp Co-10Cr-14Pt-7SiO₂ 1 20 none Example 1 Example 5Co-10Cr-14Pt-12SiN 2 10 none Example 6 Co-10Cr-14Pt-12SiN 3 6.7 noneExample 7 Co-10Cr-14Pt-12SiN 3 7.1 provided Comp Co-10Cr-14Pt-12SiN 1 20none Example 2

[0105] TABLE 2 regeneration media Br δ output noise SNR Hc (Oe) (Gμm)(mVp-p) (μV) (dB) Example 1 3,142 60 0.70 24.3 22.7 Example 2 3,272 630.71 22.5 22.9 Example 3 3,290 59 0.69 22.0 23.2 Example 4 3,492 60 0.6718.9 23.9 Comp Example 1 3,088 59 0.71 29.2 21.7 Example 5 2,982 61 0.7026.4 22.2 Example 6 3,146 59 0.69 24.1 22.5 Example 7 3,331 60 0.70 20.223.3 Comp Example 2 2,765 62 0.72 31.4 21.2

[0106]FIG. 3 shows coercive force Hc of each of the magnetic recordingmedia of Examples 1 through 4 and Comparative Example 1. FIG. 4 showsSNR (signal-to-noise ratio) of each of the magnetic recording media ofExamples 1 through 4 and Comparative Example 1. The dependence of Hc andSNR on the number of magnetic layer components was studied.

[0107]FIGS. 3 and 4, along with Table 2, show that the coercive force Hcand the SNR increase with the increase of the number of the magneticlayer components.

[0108] The Hc has been enhanced by more than 50 Oe and over 400 Oe atmaximum, and the SNR has been improved by more than 1.0 dB and by 2.2 dBat maximum in the magnetic recording media having magnetic layer 3 thatis laminated with two or more magnetic layer components as in Examples 1through 4 in comparison with the magnetic recording medium havingmagnetic layer 3 that is composed of a single magnetic layer componentas in Comparative Example 1.

[0109] The coercive force Hc and the SNR has been further improved inthe magnetic recording medium of Example 4 in which the processes ofexposing to Ar-10%O₂ gas were added before and after depositing each ofthe magnetic layer components in comparison with Example 2 that did notexperience the process. As described earlier, the improvement resultedfrom further promotion of segregation structure of the magnetic layercaused by effective oxygen supply to the granular magnetic layer fromthe oxide layers formed. Owing to this effect, the coercive force Hc hasbeen improved by more than 200 Oe and the SNR by 1.0 dB, in the magneticrecording medium having the oxide layers in comparison with the onewithout oxide layer.

[0110] When a multi-layer structure is taken in the magnetic layer,thickness of each magnetic layer component becomes thinner. In general,when film thickness of a magnetic layer is thicker, magnetization tendsto more vertically orient. Taking multi-layer structure with total filmthickness unchanged in the present invention means lamination of aplurality of thin magnetic layer components. Consequently, in-planeorientation of magnetization is enhanced, resulting in improvement ofvarious characteristics.

[0111] Moreover, in the cases of examples 5 through 7 and ComparativeExample 2 where nitride SiN was used in place of oxide SiO₂ as thematerial for forming the non-magneticmagnetic grain boundary of themagnetic layer, similar results were obtained.

Example 8

[0112] A polyolefin substrate with smooth surface was used for asubstrate. After cleaning, the substrate was introduced into asputtering apparatus. A non-magnetic magnetic under-layer 2 havingthickness of 15 nm was formed of Cr-20 at % Mo by a dc magnetronsputtering method using a target of Mo-containing Cr alloy under anargon gas pressure of 50 mTorr without heating the substrate.

[0113] Then, a granular magnetic layer component 3 a having thickness of10 nm and the same composition as the target was formed by an RFsputtering method using a target of 71 at % Co-10 at % Cr-14 at % Ptcontaining 7 mol % of SiO₂ under an argon gas pressure of 30 mTorr.Subsequently, another granular magnetic layer component 3 b havingthickness of 10 nm and the same composition as the target was formed byusing a target of 66 at % Co-10 at % Cr-14 at% Pt containing 10 mol % ofSiO₂ under the same conditions as in the component 3 a. Thus, thedouble-layered magnetic layer 3 was formed.

[0114] A carbon protective film 4 of 10 nm thickness was deposited onmagnetic layer 3 by a sputtering method. Then, the resulted article wastaken out from the vacuum of the sputtering apparatus.

[0115] A liquid lubricant layer 5, having thickness of 1.5 nm, wasformed by applying liquid lubricant of perfluoropolyether on carbonprotective film 4.

[0116] Thus, a magnetic recording medium as shown in FIG. 1 wasproduced.

[0117] In the above-described laminating process, heating non-magneticsubstrate 1 in advance was not performed.

[0118] Table 3 shows the composition of each of the magnetic layercomponents and the number of the magnetic layer components of themagnetic layer.

[0119] In this example, the chromium alloy under-layer having a bodycentered cubic lattice is accomplished an excellent (200) orientationand the granular magnetic layer is aimed at the epitaxial growth inwhich the axis of easy magnetization aligns in (110) orientation of ahexagonal closest-packed structure.

[0120] Effects in this Example are represented by an intensity ratio ofX-ray diffraction peaks and a half-width obtained by rocking curvemeasurement that expresses dispersion of orientation at the (110) peakof the uppermost component of the granular magnetic layer 3 b. Themagnitude of the half-width reflects the extent of epitaxial growth.Large intensity ratio of the X-ray diffraction peaks indicates excellentcrystallinity and small half-width of the rocking curve shows orderlyepitaxial growth.

[0121] The intensity ratio of the X-ray diffraction peaks and thehalf-width of the rocking curve were measured on the produced magneticrecording medium. The results are given in Table 4. The intensity ratioof the X-ray diffraction peaks was measured by means of powder X-raydiffractometry, that is θ-2θ method, on an X-ray diffractometer. Thehalf-width of the rocking curve was measured with an X-ray profileobtained by moving the detector represented by 2θ, while fixing thesample holder represented by θ.

[0122] Concerning magnetic characteristics of the produced magneticrecording medium, the coercive force Hc and the product Br δ of remanentmagnetic flux density and film thickness in the horizontal direction andthe vertical direction were measured using a vibrating samplemagnetometer VSM. Concerning electromagnetic conversion characteristics,regeneration output of solitary regeneration wave TAA, and media noiseand SNR (signal-to-noise ratio) at track recording density of 120 kFCIwere measured using a GMR head on a spinning stand tester. The measuredresults are given in Table 5.

Example 9

[0123] A magnetic recording medium as shown in FIG. 1 was produced inthe same manner as in Example 8 except that a magnetic layer with threelayer structure was formed by depositing a magnetic layer component 3 ahaving thickness of 7 nm and the same composition as the target using atarget of 75 at % Co-10 at % Cr-10 at % Pt containing 5 mol % of SiO₂,depositing a magnetic layer component 3 b having thickness of 7 nm andthe same composition as the target using a target of 71 at % Co-10 at %Cr-12 at % Pt containing 7 mol % of SiO₂, and depositing a magneticlayer component 3 c having thickness of 7 nm and the same composition asthe target using a target of 66 at % Co-10 at % Cr-14 at % Pt containing10 mol % of SiO₂.

[0124] In this example, like in Example 8, the chromium alloyunder-layer, having a body centered cubic lattice, has accomplished anexcellent (200) orientation. The granular magnetic layer is aimed at theepitaxial growth in which the axis of easy magnetization aligns in (110)orientation of a hexagonal closest-packed structure.

[0125] As in Example 8, the intensity ratio of the X-ray diffractionpeaks and the half-width of the rocking curve were measured on theproduced magnetic recording medium. The results are given in Table 4.The coercive force Hc in the horizontal and vertical directions, theproduct Br δ of remanent magnetic flux density and film thickness, andSNR were measured as in Example 8. The results are given in Table 5.

Comparative Example 3

[0126] A magnetic recording medium was produced in the same manner as inExample 8 except that the magnetic layer consisting of a single layerhaving thickness of 20 nm and the same composition as that of the targetwas formed using the target having composition of 66 at % Co-10 at %Cr-14 at % Pt containing 10 mol % of SiO₂.

[0127] As in Example 8, the intensity ratio of the X-ray diffractionpeaks and the half-width of the rocking curve were measured on theproduced magnetic recording medium. The results are given in Table 4.The coercive force Hc in the horizontal and vertical directions, theproduct Br δ of remanent magnetic flux density and film thickness, andSNR were measured as in Example 8. The results are given in Table 5.TABLE 3 magnetic layer composition number of layers Example 8Co-10Cr-12Pt-7SiO₂ 2 Co-10Cr-14Pt-10SiO₂ Example 9 Co-10Cr-10Pt-5SiO₂ 3Co-10Cr-12Pt-7SiO₂ Co-10Cr-14Pt-10SiO₂ Comparative Example 3Co-10Cr-14Pt-10SiO₂ 1

[0128] TABLE 4 peak intensity ratio rocking curve I(110)/I(101)half-width (deg) comment Example 8 2-3 12 (110) preferentialorientation: incomplete epitaxy Example 9 >10  4 strong (110)orientation: excellent epitaxy Comparative <<1 30 random orientation:Example 3 not epitaxial

[0129] TABLE 5 horizontal vertical Br δ Br δ SNR Hc (Oe) (Gμm) Hc (Oe)(Gμm) (dB) Example 8 3,198 61 989 11 22.7 Example 9 3,387 60 53 4 23.7Comparative Example 3,042 59 2790 25 21.6 3

[0130] It can be understood from Table 4 that the alignment in themagnetic layer of Comparative Example 3, which has a granular magneticlayer of a single layer, is random alignment that does not exhibit anypreferential orientation. The large half-width ofthe rocking curveimplies that epitaxial growth in the (200) direction of the chromiumalloy under-layer did not develop.

[0131] In longitudinal recording, c-axis of the magnetic layer havinghexagonal closest-packed structure is desired to align with in-planeorientation. In general, when alignment of the under-layer chromiumalloy having a body centered cubic lattice exhibits preferredorientation in (200) plane, the in-plane orientation in which the axisof easy magnetization of the magnetic layer is hcp (110) plane, growsepitaxially.

[0132] In the double-layered granular magnetic layer of Example 8, thelower magnetic layer component has a composition in which grain size issmaller and misfit is also smaller than those of the upper magneticlayer component. Consequently, lattice matching with upper and lowerinterfaces is improved with the aid of the lower magnetic layercomponent. The orientation of the upper magnetic layer component becamesuch that the axis of easy magnetization of Co is hcp (110) preferentialorientation. Besides, the half-width of the rocking curve indicatesdevelopment of much more improved epitaxial growth than in

Comparative Example 3.

[0133] In Example 9, one more magnetic layer component that hascomposition with small grain size and little misfit, was deposited ascompared with Example 8. Because the Example 9 allows to more preciselycontrol promotion of the lattice matching, the axis of easymagnetization in the uppermost magnetic layer component strongly alignsin hcp (110) orientation. Moreover, the half-width of the rocking curveobtained was very small, which shows a remarkable effect of themulti-layer structure.

[0134] As can be seen from Table 4 and Table 5, alignment of themagnetic layer is not controlled in Comparative Example 3 and the Coc-axis orientation is in random orientation. Consequently, the coerciveforce in vertical direction is large and the grain boundary segregationstructure is insufficient, which results large media noise anddeteriorated SNR.

[0135] In contrast, Examples 8 and 9, in which epitaxial growth from theunder-layer progressed to a considerable extent, exhibited a very smallvalue of the coercive force in vertical direction. This also reveals aneffect of multi-layer structure of the magnetic layer. The results arereduction of media noise due to promotion of the segregation structureand enhancement of SNR due to reduction of the noise caused by thevertical component of magnetization.

Effect of the Invention

[0136] By laminating a plurality of magnetic layer components eachhaving granular structure, alignment of the magnetic layer is possibleto be controlled. The resulting magnetic layer exhibits excellentcrystallinity with little dispersion of orientation. As a result, higherHc and lower noise are achieved more favorably than in a magneticrecording medium having a continuously deposited magnetic layer, incomparison with a conventional granular magnetic layer consisting of asingle continuous layer. In addition, since high Hc is easily attainedthanks to the effect of the invention even if the amount of platinum inthe target for the magnetic layer is decreased, low noise can beattained with less amount of platinum, which means possibility of costreduction.

[0137] The above-described effect produced by the multi-layer structureof the magnetic layer is enhanced when the non-magneticmagneticnon-magneticmetallic substance used in the granular magnetic layer isoxide(s) of at least one element selected from the group consisting ofCr, Co, Si, Al, Ti, Ta, Hf and Zr, when a CoPt alloy containing at leastan element selected from the group consisting of Cr, Ni and Ta is usedfor the ferromagnetic crystal in the granular magnetic layer, or whenchromium or chromium alloy is used for the non-magneticmagneticunder-layer.

[0138] By employing the multi-layer structure in the magnetic layer,even if heating of the substrate is omitted in laminating a medium ofthe invention, the media noise due to the magnetic grain size and thegrain boundary segregation and the noise due to vertical component ofmagnetization are reduced, and high Hc is readily achieved. Accordingly,plastics with low cost may be used for the substrate in addition toconventional aluminum and glass substrate.

[0139] Having described preferred embodiments of the invention withreference to the accompanying drawings, it is to be understood that theinvention is not limited to those precise embodiments, and that variouschanges and modifications may be effected therein by one skilled in theart without departing from the scope or spirit of the invention asdefined in the appended claims.

What is claimed is:
 1. A magnetic recording medium comprising: anon-magneticmagnetic under-layer, a magnetic layer, a protective film,and a liquid lubricant layer sequentially laminated on anon-magneticmagnetic substrate, wherein said magnetic layer has alayered structure including at least two magnetic layer components, eachof said magnetic layer components having ferromagnetic grains andnon-magneticmagnetic grain boundaries surrounding said ferromagneticgrains.
 2. The magnetic recording medium according to claim 1, whereinsaid at least two magnetic layer components have a composition differentfrom one another.
 3. The magnetic recording medium according to claim 1,wherein said non-magneticmagnetic grain boundaries are composed of anoxide or a nitride of at least one element selected from the groupconsisting of Cr, Co, Si, Al, Ti, Ta, Hf and Zr.
 4. The magneticrecording medium according to claim 1, wherein said under-layer iscomposed of chromium or chromium alloy.
 5. The magnetic recording mediumaccording to claim 1, wherein said substrate is composed of one of acrystallized glass, a chemically strengthened glass and a plastic.
 6. Amagnetic recording medium comprising: a non-magneticmagneticunder-layer, a magnetic layer, a protective film, and a liquid lubricantlayer sequentially laminated on a non-magneticmagnetic substrate,wherein said magnetic layer includes at least two magnetic layercomponents and at least three oxide layers; and said magnetic layercomponents and said oxide layers are alternately disposed such that atop layer and a bottom layer in said magnetic layer are two of said atleast three oxide layers.
 7. The magnetic recording medium according toclaim 6, wherein each of said magnetic layer components havingferromagnetic grains and non-magneticmagnetic grain boundariessurrounding said ferromagnetic grains.
 8. The magnetic recording mediumaccording to claim 7, wherein said non-magneticmagnetic grain boundariesare composed of an oxide or nitride of at least one element selectedfrom the group consisting of Cr, Co, Si, Al, Ti, Ta, Hf and Zr.
 9. Themagnetic recording medium according to claim 6, wherein said under-layeris composed of chromium or chromium alloy.
 10. The magnetic recordingmedium according to claim 6, wherein said substrate is composed of oneof a crystallized glass, a chemically strengthened glass and a plastic.11. A method for manufacturing a magnetic recording medium comprising:laminating a non-magneticmagnetic under-layer on a non-magneticmagneticsubstrate; laminating a magnetic layer on said under-layer by depositinga plurality of magnetic layer components, each of said magnetic layercomponents comprising ferromagnetic grains and grain boundariessurrounding said grains; laminating a protective film on said magneticlayer; and laminating a liquid lubricant layer on said protective film.12. The method for manufacturing a magnetic recording medium accordingto claim 11, wherein said step for laminating said magnetic layer is astep of depositing a plurality of magnetic layer components, eachcomponent having different composition from one another.
 13. The methodfor manufacturing a magnetic recording medium according to claim 11,wherein each step is performed without heating said substrate inadvance.
 14. A method for manufacturing a magnetic recording mediumcomprising: laminating a non-magneticmagnetic under-layer on anon-magneticmagnetic substrate; laminating a magnetic layer on saidunder-layer by (i)exposing to an atmosphere of oxygen-containing gas andforming an oxide layer on a surface, (ii) depositing a magnetic layercomponent comprising ferromagnetic grains and grain boundariessurrounding said grains, (iii) repeating the procedures (i) and (ii)predetermined times, and (iv) exposing to said atmosphere ofoxygen-containing gas and forming an oxide layer on a surface;laminating a protective film on said magnetic layer; and laminating aliquid lubricant layer on said protective film.
 15. The method formanufacturing a magnetic recording medium according to claim 14, whereineach step is performed without heating said substrate in advance.